The above-mentioned communication system is used in a situation where it is necessary to keep transmission subject data transmitted between two communication apparatuses confidential to a third party. Although various encryption techniques are proposed and used in order to keep the transmission subject data confidential, it is difficult to completely prevent decryption of a code.
In general, communication performed by encrypting the transmission subject data is performed by using a procedure of cutting the transmission subject data by a predetermined number of bits on a transmitting and receiving apparatus on a transmitter side, encrypting each piece of the cut data and transmitting it to the communication apparatus at the other end and decrypting received data on the transmitting and receiving apparatus on a receiver side.
In this case, the transmitting and receiving apparatus on the transmitter side and the transmitting and receiving apparatus on the receiver side perform encryption or decryption processing in fixed units, such as 8 bits or 16 bits.
Therefore, in the case where the size of such units are known to a third party, there is a higher possibility that an algorithm used for the encryption is broken and the contents of the communication are consequently broken.
An existing apparatus performs the encryption or decryption in standardized units, such as 8 bits or 16 bits. Therefore, in the case where the transmitting and receiving apparatus performs the encryption or decryption in the same units as such standardized units, it becomes easier to break the algorithm used for the encryption.
An object of the present invention is to improve the communication system including two communication apparatuses capable of encrypting the transmission subject data in plain text to render it as encrypted data and then transmitting it to the communication apparatus at the other end and also decrypting received encrypted data and rendering it as the transmission subject data so as to reduce the possibility of being broken by a third party.